60 LESSONS IN CHEMISTRY. 



are, however, distinguished from those compounds by con- 

 taining a less amount of oxygen.* 



Vegetable Acids. The sour or acid principles contained in 

 plants are numerous; some of them contain merely carbon 

 and oxygen, while others consist of these elements in union 

 with hydrogen in various proportions. In the living plant 

 these acids are united with various ingredients derived from 

 the soil, and when the plant is burned they are decomposed, 

 producing carbonic acid, and the alkaline and earthy substance* 

 with which they were combined are discovered in the ash in 

 the form of carbonates. We have examples of vegetable 

 acids in the acid of vmegar (acetic acid), which exists in the 

 juice of several plants; in the acid of apples (malic acid) in 

 the acid of the cuckoo son-el (oxalic acid), in the acid of 

 grapes (tartaric acid), and in the acid of lemons (citric acid). 

 These compounds, however, are of so little practical importance 

 to the ordinary farmer, that it would be out of place to 

 describe their properties. 



79. Such then, are the forms into which plants convert the 

 crude materials suppHed to them by Nature. The great mass of 

 all vegetables, as well as of the nutritious substances formed 

 within their structure, consists, as we have seen, of carbon, 

 derived from the carbonic acid of the atmosphere. This ele- 

 ment, which the air that surrounds the growing plant is at all 

 times capable of conveying to it in unUmited quantity, simply 



* The quantity of oil and fatty matter contained in our cultivated 

 plants has not, until lately, received much attention from chemists. 

 Professor Johnston, in his lectures, gives the following as the results of 

 some analyses of the flour from seven samples of wheat grown by Mr. 

 Burnett, of Glenarm, County of Antrim, at Gadgirth in Ayrshire. 

 The results obtained show, that the proportion of oil, as of the other 

 substances contained in our crops, is materially influenced by soil and 

 cultivation: — 



Oil per cent. 



1. From the undressed soil 1*4 



2. Dressed with Guano and Wood Ash . . 1*9 

 3. Artificial Guano and Wood Ash . 2-2 



4. Sulphated Urine and Wood Ash 2-2 



5, — . , Sulphate of Soda 2 -0 



6. Common Salt 2-7 



7. Nitrate of Soda 2-3 



But the proportion of oil in the flour is much lesii than in the entire 

 grain: thus, while a sample of grain gave Professor Johnston, in the 

 first flour, only 1^ petr cent of oil, he obtained from the bran above 3 

 per cent. 



